This is a tough one! But I think I am going to go with isolations, even though I haven't mastered isolations completely, they just feel great and also baffle people watching.
Mine is split time hip/shoulder reels. I just love the way it looks and the feel, also its so easy to go into some other move without it looking to stressfull.
I'm gonna have to go with basic flower and antispin flower turns. They just look so free and flow together so well, although I agree with Lulu - isolations are epic (if difficult!).
i have only been learning for a couple of months but i have to say the fountain (just mastered it - yay!!!!!!) but it was my favourite a long tome before i could even do a backwards weave. i think it looks really elegant the way it switches between weaves, so elegant in fact that non poi spinners dont know its 3 weaves linked together - it looks like a completley different move!!! i am just starting out with icolations, and have given up for a while on the behind the back weave - just until the bruises on the back of my legs have healed - then i will have another go!!!
my favourite move(s) with poi right now are stalls into plane changes and floaters!!
i love the plane changes/3D spinning cause they open up so much space in between the moves that i already have, which then makes free styling, learning and creating new moves easier, it opens up so much you will never have time for anything else.
floaters are ace because it feels like you are slowing down time, and it adds a lot to a performance.
The first time I linked/transitioned from a forwards 3 beat weave to a backwards 3 beat weave properly was an incredibly satisfying experience. I still love that, but with the fountain and turning as well it just keeps getting more exciting